A radio signal transmitted by an antenna of a communicating entity, referred to as an antenna signal, is subjected to deformation as a function of propagation conditions between a point of origin defined at the outlet of the originating antenna and a destination point defined at the inlet of an antenna of the destination communicating entity. In order to limit such deformation, the antenna signal is previously distorted by applying pre-equalization coefficients as a function of the characteristics of the propagation channel between the two antennas. It is therefore necessary to characterize the propagation channel.
Amongst existing pre-equalization methods, there are methods that are based on time reversal that are distinguished by their low complexity and their high performance.
Time reversal is a technique for focusing waves, typically soundwaves, and it relies on the invariance of the wave equation to time reversal. Thus, a wave that is reversed in time propagates like a forward wave going backwards in time.
A short pulse transmitted from a point of origin propagates through a propagation medium. A portion of this wave as received by a destination point is time reversed before being returned into the propagation medium. The wave converges towards the point of origin where it re-forms a short pulse. The signal picked up at the point of origin is practically identical in terms of form to the originating signal transmitted from the point of origin. In particular, the time-reversed wave converges with accuracy that increases with increasing complexity of the propagation medium. The time reversal of the propagation channel as applied to the wave enables energy to be concentrated and cancels out the effect of the channel on a focal point when the wave as pre-distorted in this way is transmitted from the point of origin.
The time-reversal technique is thus applied to radio communications networks in order to cancel out the effect of the propagation channel on the antenna signal at a focal point, in particular by reducing the spreading of the channel by concentrating energy on the focal point, thereby simplifying the processing of symbols that are received after passing through the channel. The antenna signal transmitted by an antenna of the originating communicating entity is thus pre-equalized by applying coefficients obtained by time reversal of the impulse response of the propagation channel through which the antenna signal is to pass. Implementing time reversal thus requires the originating communicating entity to have knowledge about the propagation channel in the frequency band that is dedicated to communication coming from that entity.
However, when using transmission in FDD mode, transmission from a communicating entity referred to as an originating communicating entity to a destination communicating entity, and transmission in the opposite direction take place in distinct frequency bands. For example, in a radio communications system, this involves transmission in a first frequency band from a mobile radio terminal to a base station, said to be transmission in the “up” direction, and transmission in a second frequency and from a base station to a mobile radio terminal, said to be transmission in the “down” direction. Although a communicating entity can estimate a propagation channel on the basis of receiving a signal that has passed through the channel, it cannot estimate a propagation channel on the basis of a signal that has been transmitted in a different frequency band.
No reciprocity property concerning the transmission channel can be applied, unlike transmission in time division duplex (TDD) mode where sharing the same frequency band makes it simple to estimate the channel independently of transmission direction. It is therefore advantageous to have a technique for pre-equalizing antenna signals for use with transmission in FDD mode.
A first solution is proposed in the article entitled “From theory to practice: an overview of MIMO space-time coded wireless systems” by David Gesbert, Mansoor Shafi, Da-Shan Shiu, Peter J. Smith, and Aymon Naguib, and published in IEEE Journal on Selected Areas in Communication, Vol. 21, No. 3, in April 2003. The method proposed relies on time reversal as a pre-equalization technique in which the coefficients are evaluated from an estimate of the propagation channel performed by the destination communicating entity. The estimation is performed by the destination communicating entity on the basis of knowledge of pilots previously transmitted by the originating communicating entity. A quantification of the estimate of the propagation channel is then delivered to the originating communicating entity. Nevertheless, the quantification of the estimate of the propagation channel needs to be sufficiently efficient in order to guarantee that pre-equalization is effective. Furthermore, the radio resources needed for delivering the quantified estimate of the propagation channel increase with increasing accuracy of the quantification.
A compromise therefore needs to be reached between the accuracy of the quantification of the estimate of the propagation channel and consumption of radio resources used for transmitting the quantified estimate of the propagation channel.